INDIANAPOLIS — The Green Bay Packers were up 18 points at halftime and apparently ready to cruise to a comfortable win over the rebuilding Indianapolis Colts on Sunday.

But a second-half offensive collapse combined with the play of the Colts’ plucky rookie quarterback, Andrew Luck, and a missed field goal in the final seconds meant the favored Packers left Lucas Oil Stadium with a 30-27 loss to the young Colts.

So after one bad half of football, the Packers suddenly are 2-3, and if not reeling, must be wondering what’s happened to the offensive powerhouse that cruised through 2011, putting up points almost at will.

“They’re hard to win, even harder to win on the road,” Packer coach Mike McCarthy said. “When you’re 21-3 at halftime, you expect to keep that thing going. That was our approach when we came out (of halftime). We had a couple of three-and-outs, and it was kind of downhill from there.”

The dispiriting defeat was the fourth-largest blown lead in Packers history, behind a 22-pointer against the Los Angeles Rams in 1952, and 21-pointers against the Rams in 1957 and Atlanta Falcons in 1983.

“When you have a team on the ropes like we did, you have to take care of it,” cornerback Tramon Williams said. “We didn’t. It’s kind of been like that all season. We were at a crossroads in the New Orleans game (last week) where we could have taken over and made those guys play a totally different way and we didn’t, we left them in the game. We just have to put these teams away.”

For all that went wrong in the second half, the Packers had a chance to send the game into overtime, but a crucial game-management error and shank on a long field-goal attempt cost them the tie in the waning seconds.

The game-management mistake was having to burn their final timeout when the play clock was about to expire with the ball at the Colts’ 33. The game clock was stopped with 8 seconds remaining because quarterback Aaron Rodgers had spiked the ball the previous snap, but the Packers were too slow calling the play and changing the protection at the line of scrimmage, so Rodgers had to call his final timeout.

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That meant the Packers had to try the game-tying field goal from 51 yards rather than run a play to get a little closer for kicker Mason Crosby. Crosby shanked the field goal, hitting what he described as a knuckleball that started straight but quickly veered left.

“With what happened, it doesn’t matter (if the kick had been shorter),” Crosby said. “It wasn’t anything about that. It wasn’t the range. I was trying to hit a good ball. How it came off, I guess I hit it high on my foot and it just knuckled off.”

Now nearly a third of the way through the season, the Packers come out of Week 5 a little battered, mentally and physically.

A team that lives off its playmaking talent in its passing game hadn’t been producing even when healthy, and now its depth looks shaky after it ended up playing much of this game without its two best receiving threats and its starting halfback.

The Packers went into Sunday without receiver Greg Jennings, who appears likely to miss several more games because of a groin injury. Then they lost starting halfback Cedric Benson to a foot injury of undetermined severity early in the second quarter and tight end Jermichael Finley to a shoulder injury early in the third quarter.

Still, they had Jordy Nelson, James Jones and Randall Cobb at receiver, and Alex Green, a third-round draft pick last year, at running back. With the reigning MVP in quarterback Aaron Rodgers, the Packers’ offensive collapse in the second half, when they scored only six points against a team missing two of its top three cornerbacks, was stunning.

“They were very aggressive with us,” McCarthy said. “That’s the M.O.: Jump up and challenge the Packer perimeter (the receivers). That’s going to be the M.O. until we start gashing people, and we didn’t do it today.”

The rookie Luck, the first pick in this year’s draft, arguably played Rodgers at least to a draw and led his team to the game-winning touchdown in the game’s final 4½ minutes.

Though Luck’s passer rating was mediocre — 81.0, to Rodgers’ 103.5 — he threw for more yards (362 to Rodgers’ 243) and had the same number of interceptions (one). Though Luck at times made risky throws into traffic, the Packers didn’t make him pay, and he regularly made good plays out of bad.

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Luck showed the ability to escape the rush with either his feet or his strength, ran when he had to (six carries for 24 yards) and displayed outstanding chemistry with veteran receiver Reggie Wayne. Luck targeted Wayne 20 times, and they connected on 13 for 212 yards, which is the second-most receiving yards the Packers ever have allowed an opposing player.

One of Luck and Wayne’s biggest plays came on the game-winning drive, when on a third-and-12 from the Packers’ 47 the quarterback appeared about to be sacked by outside linebacker Clay Matthews. But the 6-foot-4, 234-pounder stayed upright and muscled a 15-yard completion to Wayne over the middle to keep the drive alive.

“Big kid,” Matthews said, “elusive, slippery. Unfortunately there wasn’t much pressure in his face to get him back on his heels. But I just didn’t get him down, ultimately that’s what it boils down to. Big kid, very elusive, and he can make plays.”

The Packers have given up more than 400 yards in back-to-back games — last week, New Orleans had 474 yards, and the Colts had 464. Those numbers are reminiscent of last season, when they finished with the No. 32 defense in the league in yards allowed.

But the Packers’ offensive meltdown was just as responsible. Colts cornerback Jarraud Powers set up his offense on a short field early in the third quarter when he intercepted Rodgers’ back-shoulder pass to James Jones at the Packers’ 39. Rodgers thought the ball was tipped at the line of scrimmage; replays were inconclusive.

“We just stopped gaining ground, and we couldn’t get any points on the board, we just came to a halt,” Williams said. “Any time you do that, then another team is chipping away, chipping away, they continue their same game plan, you don’t quite knock them out of their game plan.”

Defensive lineman Ryan Pickett said: “It isn’t so much that we’re 2-3, it’s how we’re 2-3. We’re not playing good, we didn’t get better. Just an all-around bad performance. Everything. It was bad.”